In producing a sensor electrode, a heater electrode and the like for various gas sensors such as an oxygen sensor, a NOx sensor, and an exhaust temperature sensor, these electrodes are generally produced by coating a metal paste including a conductive metal powder on a substrate by various methods such as a screen printing method and calcining it. The reasons for frequently using a form of a metal paste are that the metal paste can be adapted to a complex electrode pattern, and furthermore that coating and calcining the metal paste on a green sheet forming a ceramic substrate allow a substrate and an electrode to be produced simultaneously, which is preferable from the viewpoint of production efficiency.
A metal paste for electrode formation, which has been conventionally used is one that is obtained by mixing a conductive particle such as a precious metal and a ceramic powder such as Al2O3 or ZrO2 (YSZ) with a solvent. The reason for mixing a ceramic powder in a metal paste is that when the metal paste is coated on a green sheet and calcined to produce a substrate and an electrode as described above, the difference in shrinkage rate between the metal paste and the green sheet is so modified as to solve problems for warpage and deformation of a substrate due to the difference in shrinkage rate, thereby improving adhesion of an electrode. In addition, there is also an advantage that the mixing of a ceramic in a metal paste could prevent a conductive particle from being excessively sintered upon calcining.
On the other hand, since a metal paste for electrode formation is a precursor material for an electrode, it is naturally required to be low in electric resistance value (specific resistance). However, the mixing of a ceramic powder is an obstructive factor in terms of lowering the resistance of an electrode, and there has been a tendency that the resistance value of an electrode film formed by calcining a metal paste would be significantly higher than that of a bulk metal. That is, although the improving of adhesion with mixing a ceramic in a metal paste is contrary to a demand of lowering the resistance of an electrode, no or quite a little ceramic to be mixed makes an electrode formation itself become impossible.